Generally, an oscillating electric motor is constructed as a system capable of maintaining oscillations, to which energy is supplied by means of a coil operated as an electromagnet. In order to ensure that the proper amount of energy is supplied at the proper timing, it is helpful to detect the state of motion of the electric motor. In this regard, a variety of approaches have been described in the art.
U.S. Pat. No. 5,632,087 describes a dry shaver with a linear motor. The linear motor includes a stator winding and a reciprocator which is equipped with a permanent magnet and is set in a linear oscillation by the stator winding. The displacement of the reciprocator is sensed by means of a detector and, depending on the sensed displacement, the supply of current to the stator winding is controlled such that the oscillation amplitude of the reciprocator is maintained constant. The detector is comprised of a permanent magnet arranged on the reciprocator and a fixedly mounted sensor winding in which, through action of the permanent magnet, an induced voltage is generated. The induced voltage is dependent upon the speed of the reciprocator.
EP 1 063 760 A2 discloses a control system for a linear vibration motor in which a vibrator is set in a linear vibration with a constant vibration amplitude relative to a stator. The linear motor includes a winding arranged on either the stator or the vibrator. A permanent magnet is arranged on the respective other component (i.e., on the vibrator or on the stator). The magnetic field generated when current is transmitted through the winding acts on the permanent magnet, causing the vibrator to be set in a linear vibration. In this process, the current flow is controlled to produce a constant vibration amplitude. To determine the vibration amplitude, the current supply to the winding is interrupted and the voltage then developed across the winding is determined.